There have been a number of prior art processes attempting to recover soda ash and/or bicarb from nahcolite deposits. We have now found that the process of producing these compounds by solution mining of nahcolite deposits at high temperatures is more economical and has a number of other advantages. In particular, the present invention involves the in situ solution mining of nahcolite using high temperature (i.e., above 250° F.), high pressure water and the subsequent processing of the production solution to produce soda ash and bicarb. The invention enables bicarb and soda ash to be economically recovered, not only from beds of virtually pure nahcolite, but also from oil shale containing much smaller amounts of nahcolite embedded therein.
Prior to this invention, no one had appreciated the benefits that could be achieved through the high temperature solution mining of nahcolite and the recovery of sodium carbonate and sodium bicarbonate. Indeed, prior art processes utilized solution mining temperatures below 250° F. and preferably below 200° F. (See, e.g., U.S. Pat. No. 4,815,790 to Rosar et al. and assigned to NaTec, Ltd.). Others advocated adding acid to the injection solution to effect a reaction in the ore body that produced an alkali species with higher solubility. For example U.S. Pat. No. 4,475,771 (assigned to Duval Corp.) advocated the use of hydrochloric acid, and U.S. Pat. Nos. 4,358,158 and 4,358,157 (assigned to Union Oil) encouraged the use of carbonic acid. The present invention does not require the use of acids or caustic materials; for solution mining of nahcolite.
Although U.S. Pat. No. 3,779,602 (assigned to Shell Oil Company) disclosed the possible use of low pressure steam at temperatures in excess of 250° F. and preferably in excess of 300° F., published test reports indicate that the process was not successful. The Shell process, which was designed to recover oil as well as nahcolite, resulted in solids precipitation causing plugging and “flow impairment.” (See, e.g., M. Prats et al. “Soluble-Salt Processes for In-Situ Recovery of Hydrocarbons From Oil Shale,” Journal of Petroleum Technology, 1078-88 (September, 1977).) The steam caused too much fracture of the oil shale formation and had too little water content to adequately dissolve the nahcolite. Moreover, the process was designed to decompose and recover hydrocarbons from the oil shale, impurities that would make it substantially more difficult and expensive to recover soda ash and/or bicarb from the well production. Accordingly, commercial solution mining of nahcolite has traditionally been conducted at ambient or slightly elevated temperatures. For these reasons, steam is not used in the present process.
Prior to the present invention, it was generally believed that any increase in nahcolite solubility at elevated temperatures was so minimal that it did not justify the additional energy requirements associated with high temperature recovery of nahcolite. (See, Waldeck et al., “Aqueous Solubility of Salts at High Temperatures,” 54 J. Am. Chem. Society 928 (March 1932) and Waldeck et al., “Aqueous Solubility of Salts at High Temperatures,” 56 J. Am. Chem. Society 43 (January 1934) which report data for the solubility of nahcolite up to 392° F.) Indeed, the Waldeck data was generally believed to be the most authoritative statement on the solubility of nahcolite and its related carbonate compounds. Surprisingly, our work has demonstrated a much higher solubility for nahcolite at temperatures above 250° F.
It has now been discovered that the solubility of bicarb at elevated temperatures is much higher than that reported by Waldeck or predicted by extrapolating the Waldeck data above 392° F. For example, the published data indicates a solubility of about 27% for bicarb at 300° F., when in fact, the solubility is about 32% at that temperature. (The solubility percentages identified herein are based on weight.) The differences between Waldeck's published solubility information and the solubility we have found is illustrated in FIG. 5. As illustrated in FIG. 5, Waleck's solubility curve and the actual solubility curve that we have found diverge, so that at higher temperatures, the difference is even greater. This discrepancy in the published and actual solubility has been confirmed by actual solution mining tests.
This surprising discovery led to the development of the present processes which permits (through the use of higher concentrations) the more efficient mining and production of alkali from nahcolite. As used herein, the term “alkali” refers to the total sodium carbonate and sodium bicarbonate. Indeed, up to this time no one had contemplated the economical solution mining of nahcolite at the temperatures and pressures described herein.
Also, contrary to prior perceptions, it has been found that solution mining may be successfully conducted within nahcolitic oil shale intervals which contain nominally-horizontal beds of pure nahcolite and in intervals which contain both nodular and bedded nahcolite deposits. Shale fracturing can be controlled in either of these type of deposits to facilitate the solution mining of the nahcolite.
Contrary to published information (M. Prats, et al., supra), it has been found that the use of a hot aqueous phase solution mining process does not result in excessive fracturing and spalling. Solution mining of nahcolite in accordance with the present invention does not cause excessive “rubbling,” i.e., disintegration of the shale rock into smaller pieces, which causes plugging of the mining cavity or the production tubing.
The present invention also utilizes pressure within the solution mining zone to prevent flashing of the mine solution, i.e., steam and carbon dioxide. Such flashing could potentially cause the decomposition of sodium bicarbonate and/or precipitation of the dissolved salts. To prevent flashing the mining zone is maintained at pressures up to the minimum hydraulic fracture pressure of the deposit being mined. This can be achieved by employing a pressurized cap of inert gas above the area of solution mining, as described more fully later, to maintain the pressure in the mining zone. Preferably, this blanket of inert gas is fed down the annulus between the injection pipe and casing. The pressure utilized for this inert gas cap is above that required to prevent flashing of the mining solution but below the minimum hydraulic fracture pressure. Alternatively, the pressure control can be accomplished by restricting the production solution flow.
Because the production solution exiting the mine is at high temperatures and pressures, the downstream processing is also novel. While some prior art processes added heat in order to decompose bicarb, the prior art did not involve high temperature and high pressure environments as used in the present invention. With the present process, the temperature of the production solution recovered from the mining zone is above that of subsequent processing and, therefore, creates much of the driving force for the decomposition. In fact, by staging the decomposition, the driving force can be spread over at least four stages in series which can include flashing, first stage decomposition, second stage decomposition, and evaporation. Bicarb decomposition occurs in all of these stages. One of the unique aspects of this processing is that three or all four stages are accomplished under pressure. “Conventional wisdom” would not consider this processing practical, since the use of high pressure would make it harder to decompose bicarb and release the carbon dioxide out of solution.
It has also been discovered that the high temperature production solution may contain a relatively large quantity of sodium carbonate without appreciably altering the total alkali being carried by the solution. Again, the prior art (such as U.S. Pat. No. 4,815,790) teaches that the concentration of sodium carbonate should be limited or controlled (i.e., by keeping temperatures below 250° F.) to facilitate dissolution of the deposit and/or to prevent downstream operational costs and other problems.
The foregoing are but a few of the differences and advantages which the present invention has exhibited over the prior art. Other objects and advantages will become apparent to one skilled in the art from the description of the invention and drawings contained herein.